1. Field of the Invention
The present invention relates to a tunable thin film capacitor with a thin film dielectric layer whose dielectric constant is changed by the voltage applied across a lower electrode layer and an upper electrode layer.
2. Description of the Related Art
It is so far known that a strontium titanate (SrTiO3) thin film that is a paraelectric substance or a strontium barium titanate (Ba, Sr) TiO3 thin film that is a ferroelectric substance exhibits a non-linear change in the dielectric constant thereof upon application of a predetermined voltage to the dielectric thin film thereof (A. Walkenhorst et al., Appl. Phys. Lett. 60 (1992) 1744 and Cem Bascriet. al., J. Appl. Phys 82 (1997) 2497).
Thin film capacitors using as their thin film dielectric layers perovskite structure ferroelectric oxide thin films of these strontium titanate and strontium barium titanate, etc. are proposed (Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
These tunable thin film capacitors are each constituted in such a manner that, on a support substrate 161, a lower electrode layer 162, a thin film dielectric layer 163 and an upper electrode layer 164 are successively formed by deposition as shown in a sectional view of FIG. 24. More specifically, on the support substrate 161, a conductor layer that will become the lower electrode layer 162 is formed by deposition over approximately the whole surface of the substrate 161, and then, patterning is performed to form the lower electrode layer 162 of a predetermined shape. Next, the dielectric layer 163 is formed on the lower electrode layer 162. This dielectric layer 163 is formed, by the thin film method, with a mask placed at a predetermined position, or formed by the spin coating process, and then patterned into a predetermined shape. Further, the layer is heated for hardening as required. The upper electrode layer 164 is formed in such a manner that a conductor layer that will become the upper electrode layer 164 is formed on the dielectric film 163 and then pattern-processed. Here, the facing region of the dielectric layer 163 which is actually held between the lower electrode layer 162 and the upper electrode layer 164 becomes a capacitance-producing region.
In the dielectric layer 163 in this capacitance-producing region, the dielectric constant that the dielectric layer 163 has is changed by an external control voltage fed across the lower electrode layer 162 and the upper electrode layer 164.
Accordingly, in case that the mutually facing areas of both electrodes layers 162 and 164 and the thickness of the dielectric layer 163 are made constant, the capacitance value obtained between both electrode layers 162 and 164 can be varied by setting the external control voltage at a predetermined voltage.
Further, the thin film capacitor shown in FIG. 24 is constituted such that, in order to prevent the lower electrode layer 162 and the upper electrode layer 164 from short-circuiting to each other, the upper electrode layer 164 is extended onto the support substrate 161 so as to form an air bridge 165. In other words, by extending the upper electrode layer 164 onto the support substrate 161, as pace is formed around the dielectric layer 163.
This space can be formed in such a manner that an organic resist member that is formed by heat treatment or the like is formed, and thereafter, the upper electrode layer 164 is formed and heat-treated.
In such a variable capacitor, the capacitance thereof is changed by a control voltage applied across the lower electrode layer 162 and the upper electrode layer 164, and therefore, it is important that this voltage be uniformly applied to the dielectric layer 163.
This external control voltage is, for example, ten to several tens of volts, but this external control voltage is, actually, hard to be uniformly applied to the capacitance-producing region; and thus, it is difficult to stabilize the dielectric constant of the dielectric layer 163 to a predetermined value. For example, by taking the maximum capacitance into consideration, the facing area of the capacitance-producing region is set, but, in case that an attempt is made to obtain this capacitance value from one capacitance-producing region, the area of the capacitance-producing region is increased. As a result, the voltage drop of the control voltage occurs in the upper electrode layer 164 and the lower electrode layer 162; and, of the lower electrode layer 162 and the upper electrode layer 164, the portions adjacent to portions 162a, 164a extended from the capacitance-producing region become high in potential, while, in the central portion of the capacitance-producing region and portions away from the extension portions 162a, 164a shown in FIG. 25, the potential becomes relatively low. In other words, in the same capacitance-producing region, a distribution of potential takes place, resulting in the problem that the sufficient dielectric constant control, i.e., a sufficiently wide variable range, cannot be obtained; due to this, there has been the problem that a capacitance corresponding to the external voltage cannot be obtained as according to the specifications.
Further, in order to structurally prevent the upper electrode layer 164 and the lower electrode layer 162 from short-circuiting to each other, the air bridge 165 is provided. The existence of this hollow structure results in giving a large restriction when the capacitor is mounted on a mother board and in lacking in reliability. Further, as for the method of manufacturing the upper electrode layer 164, an organic resist is used for the formation of the air bridge 165, and thus, a manufacturing method deviating from the thin film technology must be used, so that the manufacturing steps become very complicated.
To a tunable thin film capacitor that operates in a high-frequency region, it is important to reduce the loss of the respective electrode layers. For this, it necessary to increase the thickness of a lower electrode layer 162 and an upper electrode layer 164. In practice, however, it is difficult to increase the thickness of the lower electrode layer 162.
It is because, in the case of increasing the thickness of the lower electrode layer 162, the coverage of the lower electrode layer 162 to the dielectric layer 163 is deteriorated. Further, separation by peeling takes place between a substrate 161 and the lower electrode layer 162. Moreover, separation by peeling takes place between the dielectric layer 163 and the lower electrode layer 162. These inconveniences are caused because, by increasing the thickness of the lower electrode layer 162, the stress resulting from the difference between the thermal expansion coefficients of the substrate 161 and the dielectric layer 163 is increased.
Particularly, in the case of such a variable capacitance element, the increase in thickness of both electrode layers 162, 164 is very important in view of the fact that a relatively high capacitance controlling voltage of about 10 V must be applied to the dielectric layer 163 between the lower electrode layer 162 and the upper electrode layer 164.
Further, in the case of the known tunable thin film capacitor, an air bridge 165 was formed around the dielectric layer 163 in order to prevent the upper electrode layer 164 from short-circuiting to the lower electrode layer 162. Thus, a portion of the upper electrode layer 164 is positioned in the air like a bonding air, so that it is very difficult to actually mount the capacitor onto a mother board or the like; and thus, the known tunable thin film capacitor could hardly be put to practical use.
Concerning a tunable thin film capacitor constituted such that, by application of an external voltage, the dielectric constant of the dielectric layer is changed to vary the capacitance, the important problems to be solved are to reduce the voltage loss and signal component loss (hereinafter referred to merely as loss) due to the resistance component of the electrode portions and to improve the drapeability or coverage of the dielectric layer.
However, in the case of the known structure, if the thickness of a lower electrode layer 162 is increased in order to reduce the loss of the lower electrode layer 162 and the upper electrode layer 164, the coverage of a dielectric layer 163 for the lower electrode layer 162 is deteriorated. Further, when an external voltage (driving voltage) for varying the capacitance is applied, the electric field intensity becomes the maximum in a thin portion of the dielectric layer 163 (for example, the end of that portion of the dielectric layer 163 which covers the lower electrode layer 162), whereby dielectric breakdown takes place. Or, a crack is made in the dielectric layer 163, so that the lower electrode layer 162 and an upper electrode layer 164 opposite to it are short-circuited through the dielectric layer 163.
Further, there is the problem that, in case that a crack is produced at the end of the dielectric layer 163 in the externally extended portion of the upper electrode layer 164, of the lower electrode layer 162 and the upper electrode layer 164 that face each other through the dielectric layer 163, the portion that constitutes the capacitance is opened, and thus, the designed capacitance cannot be obtained.
Among the capacitors, there is a thin film capacitor constituted such that the electrode layers and the dielectric layer being constituent elements of the capacitor are formed of thin films. In this capacitor, normally, the thin film lower electrode layer, the thin film dielectric layer, and the upper electrode layer are successively laminated on an electrically insulating substrate in this order. In such a thin film capacitor, the lower electrode layer and the upper electrode layer are respectively formed by sputtering, vapor deposition or the like, and in addition, the dielectric layer is also formed by sputtering, the sol-gel process or the like. In the manufacture of such a thin film capacitor, normally the photolithography method is employed as follows: First, a conductor layer that will become the lower electrode layer is formed over the whole surface of an insulating support substrate, only the necessary portion of the conductor layer is then covered with a resist, and thereafter, the unnecessary portion of this conductor layer is removed by wet etching or dry etching to form the lower electrode layer of a predetermined shape. Next, on the support substrate, a dielectric layer that will become the thin film dielectric layer is formed over the whole surface, and the unnecessary portion of this dielectric layer is removed therefrom as in the case of the lower electrode layer, whereby the thin film dielectric layer of a predetermined shape is formed. Finally, a conductor layer that will become the upper electrode layer is formed over the whole surface, and the unnecessary portion of this conductor layer is removed to form the upper electrode layer of a predetermined shape. Further, a protective layer and solder bumps are formed, whereby it becomes possible to surface-mount the capacitor. Further, a tunable thin film capacitor constituted such that, as the material of the thin film dielectric layer, a dielectric material comprised of (BaxSr1-x) TiO3 is used, and, by applying a predetermined potential across the upper electrode layer and the lower electrode layer to change the dielectric constant of the dielectric layer, whereby the capacitance is controlled, is also of the same structure.
In order to use a thin film capacitor as, e.g., a capacitor in a high-frequency circuit, the self-resonant frequency thereof must be positioned at the side higher than the frequency used. Such a thin film capacitor can be realized by making the inductance at the lower electrode layer and the upper electrode layer small; a thin film capacitor with a small inductance is disclosed in, e.g., Japanese Unexamined Patent Publication JP-A 8-241830 (1996).
In order to use a thin film capacitor as a capacitor in a high-frequency circuit as mentioned above, the self-resonant frequency must be positioned at the side higher than the used frequency, the inductance must be small, and in addition, the loss of the lower electrode layer and the upper electrode layer must also be low. This is because, even in case that the resonance point is positioned at the side higher than the frequency at which the capacitor is used, the impedance resulting from the capacitor is small at a frequency in the vicinity of the resonance point, so that, in a capacitor with a large loss, the resistance component becomes predominant. Therefore, in order to reduce the loss due to the lower electrode layer and the upper electrode layer, a metal with a small resistivity must be used as the material of the lower electrode layer and the upper electrode layer, and the lower electrode layer and the upper electrode layer must be made as thick as possible.
Further, by making the capacitance of the capacitor small, it becomes possible to shift the self-resonant frequency thereof further towards the high-frequency side, and thus, the increase in the loss by the influence of resonance can be reduced. In order to reduce the capacitance of the capacitor, it is necessary to decrease the area of the capacitance-producing region comprised of the thin film dielectric layer sandwiched between the lower electrode layer and the upper electrode layer, but, by decreasing the area of the capacitor, a step that deteriorates the leakage characteristic is formed in the dielectric portion constituting the capacitor, and the accuracy in positional matching becomes hard to achieve at the manufacturing steps, thus resulting in a fall in manufacturing yield.
In order to use a thin film capacitor as a constituent part of a filter, a resonator or the like in a high-frequency circuit, it is required for the capacitor to have a high Q-value. The Q-value depends on the losses in the respective constituent elements of the capacitor; the dielectric loss of the dielectric, the ohmic loss in the electrodes, etc. are the main causes for lowering the Q-value.
In order to reduce the loss due to the lower electrode layer and the upper electrode layer, it is necessary to use a metal with a low resistivity; gold, silver, copper, aluminum, etc. are efficient, but, in the case of these metals, there is the possibility that there may be caused the problem that these metals are each oxidized at a high temperature during the manufacturing steps, the film quality thereof is deteriorated, the metals are chemically reacted with the dielectric layer. For example, in the case of an electrode comprised of gold, the smoothness of the electrode is deteriorated at a high temperature, which will cause the short-circuiting of the capacitor. Due to this, the structure constituted such that, on a layer of tungsten or molybdenum that is a high-melting metal material, platinum or the like is laminated as a reaction-preventing layer has so far been used as an electrode material (See Japanese Unexamined Patent Publication JP-A 9-260603 (1997)).
Further, it is a common practice to laminate an adherent layer using Ti or Cr between the support substrate and the electrode material for the purpose of enhancing the adherence.
However, there has been caused the problem that the high-melting metal material is low in conductivity as compared with gold or the like, and, in particular, the electrode loss cannot be reduced in a high frequency region higher than 1 GHz. Further, in case that platinum is used as the material of the reaction-preventing layer, there has also been caused the problem that the thermal stress of the reaction-preventing layer is large, so that the reaction-preventing layer is apt to peel off from the interface between the lower electrode layer and the support substrate due to the change in temperature during the manufacturing steps. Further, in case that gold is used as the conductive layer of the lower electrode layer, the film quality is markedly deteriorated due to the high temperature during the manufacturing steps, and thus, there have been caused problems such as the problem that the capacitor is short-circuited and the problem that the leakage current becomes large. Further, Ti, Cr, and the like are easy to react with BST and PZT that are general dielectric materials, so that, in case that these metals are used as adherent layers, there is the possibility that the characteristic of the dielectric may be deteriorated at the time of a process using a high temperature.
In such a tunable thin film capacitor, the upper electrode layer 164 was made into an air bridge structure 165, and therefore, the electrode length thereof became large, so that the conductor loss became increased. Further, due to the curved shape which is peculiar to the air bridge structure, there was caused the problem that the self-inductance L was large, so that the self-resonant frequency, defined by the condition: fo=xc2xd (LC)1/2 wherein C represents a static capacitance, was small, and thus, the operating frequency was limited to a low frequency region.
As a variable capacitance capacitor, there has so far been used a varactor diode constituted such that, by applying a reverse bias to the diode, the capacitance is changed. A diode is used in a rectifier circuit, etc., utilizing the phenomenon that, when a bias is forwardly applied to the PN junction thereof, a current flows. In the PN junction interface, there exists a region called a depletion layer in which no electron or hole exists. When a reverse bias is applied to the diode, the electrons and holes are both pulled in the direction a way from the PN junction interface, so that the depletion layer becomes thick, and the thickness of the depletion layer changes depending on the magnitude of the reverse bias. This depletion layer can be considered to be a dielectric, so that, in case that a reverse bias is applied to the diode, the thickness of the dielectric changes depending on the magnitude of the reverse bias, as a result of which the diode can be utilized as a capacitor whose capacitance changes.
The varactor diode is standardized for the utilization thereof as a variable capacitance capacitor. The varactor diode is used as the variable capacitance capacitor in communication apparatus, but, recently, due to the increase in the demand for portable communication terminals, the frequency band used for communications is being made higher, and in addition, efforts are being made to lower the voltage at the terminals. In the varactor diode, its loss is increased at high frequencies, and, in the case of using a low voltage, the depletion layer becomes thin, and the leakage current increases; and thus, the varactor diode theoretically ceases to function as a capacitor, so that it is difficult to make the varactor diode compatible with a high-frequency circuit (compatibility with the high-frequency operation).
Due to this, (BaxSr1-x) TiO3 (hereinafter abbreviated as BST) and other dielectric materials are proposed in order to constitute an element that is usable as a variable capacitance capacitor even in a high-frequency operation (See, for example, Japanese Unexamined Patent Publication JP-A 11-3839 (1999)).
The rate of change in the capacitance of a variable capacitance capacitor having a dielectric layer made of such a dielectric material becomes a function of the electric field intensity applied to the dielectric layer, so that the thickness of the dielectric layer used in a variable capacitance capacitor whose capacitance is changed by a low voltage must normally be several xcexcm or less. In order to fabricate such a dielectric layer, methods such as the sputtering process, the sol-gel process and the CVD process are used. In order to fabricate variable capacitance capacitors comprising dielectric layers at low costs, in large quantities and stably, it is considered that the use of the sputtering process is effective. In order to fabricate the dielectric layer by the use of the sputtering process, it is a common practice to use, as the target, ceramics of the same composition as the dielectric to be obtained. In the sputtering process using such a ceramics target, it is a common practice that an RF sputtering apparatus is used, and that, as the sputtering atmosphere, a mixture resulting from adding an O2 gas into an Ar gas is used. Normally, in the case of forming a thin metal film, sputtering is performed in an atmosphere comprising only an Ar gas, but, in the case of using ceramics, the elimination of oxygen takes place at the time of film-forming deposition, as a result of which the oxygen concentration of the fabricated dielectric layer becomes less than that of the stoichiometric composition, and a large amount of lattice defects of oxygen are produced. In order to suppress the production of such lattice defects of oxygen, an O2 gas is introduced into the atmosphere at the time of sputtering. As a matter of fact, only by the alteration of the sputtering atmosphere by the introduction of an O2 gas, the formation of lattice defects of oxygen cannot be completely suppressed, and thus, in order to further reduce the lattice defects of oxygen, heat treatment is performed, after the sputtering, for a long time at a temperature higher than the substrate temperature at the time of sputtering (See, for example, Japanese Unexamined Patent Publication JP-A 9-31645 (1997)).
Further, a report is made on the method according to which, in order to suppress the deterioration in dielectric loss due to the lattice defects of oxygen, Mn or the like is added into the dielectric layer in addition to the heat treatment performed after the sputtering (See, for example, W. Chang, et al., Mat. Res. Soc. Symp. Proc. Vol. 541, (1999) 699) However, the method of suppressing the deterioration in dielectric loss due to the lattice defects of oxygen or the production itself of lattice defects of oxygen is problematic in that the method requires a heat treatment extending over a long time after the sputtering and thus poor in efficiency from the viewpoint of a method of fabricating variable capacitance capacitors with an enhanced productivity; and thus, variable capacitance capacitors cannot be fabricated with low costs.
In order to use the thin film capacitor as a capacitor in a high-frequency circuit as mentioned above, the self-resonant frequency must be positioned at the side higher than the frequency used, the inductance must be small, and further, the loss of the electrodes must also be low. This is because, even in case that the resonance point is positioned at the side higher than the frequency at which the capacitor is used, the impedance resulting from the capacitor becomes small at a frequency in the vicinity of the resonance point, so that, in a capacitor with a large loss, the resistance component becomes predominant.
Thus, in order to reduce the loss due to the electrodes (for example, the loss of the high-frequency signal component when the capacitor is used in a high-frequency circuit), a metal that has a small resistivity must be used as the material of the electrodes, and the electrodes must be made as thick as possible.
However, at the steps of manufacturing a thin film capacitor, the electrodes are formed by the thin film technique and the photolithography method, so that the manufacturing steps become complicated. Further, there is the problem that, in case the electrodes are made too thick, the electrodes peel off from the surfaces to which they are to adhere.
The present invention has been made in view of the problematic points mentioned above, and it is the object of the invention to provide a tunable thin film capacitor of such a structure that an external control voltage applied can be uniformly applied to the dielectric layer.
Another object of the invention is to provide a tunable thin film capacitor of such a structure that the manufacturing steps thereof are simplified, and yet, the upper electrode layer and the lower electrode layer are structurally hard to short-circuit to each other and suited to surface mount.
Still another object of the invention is to provide a tunable thin film capacitor constituted such that the dielectric layer does not peel off, and in addition, the resistance in the upper electrode layer of the capacitance can be easily lowered, and, even in the case a relatively high capacitance-controlling voltage is applied to the dielectric layer, the capacitor can withstand it.
Further another object of the invention is to provide a tunable thin film capacitor constituted such that, by appropriately altering the electrode structure and appropriately setting the thickness of the respective electrodes, the coverage of the dielectric layer and the coverage of the upper electrode are enhanced to prevent the short-circuiting or open-circuiting between the two electrodes, whereby a tunable thin film capacitor which allows the stable derivation of the capacitance component is provided, maintaining the productivity.
Further another object of the invention is to provide a tunable thin film capacitor constituted such that the electrode loss of the lower electrode layer and the upper electrode layer is small, and the area of the capacitance-producing region can be formed with a high accuracy.
Further another object of the invention is to provide a tunable thin film capacitor constituted such that the loss due to the electrodes is small even at a high frequency, so that the Q-value is large, and, during the manufacturing steps, the lower electrode layer is hard to peel off, and thus, the reliability is high.
Further another object of the invention is to provide at unable thin film capacitor with a high self-resonant frequency.
Further another object of the invention is to provide a tunable thin film capacitor that has a protective film with a good adherence and thus is high in reliability.
Further another object of the invention is to provide a tunable thin film capacitor that comprises a dielectric layer with less lattice defects of oxygen.
Further another object of the invention is to provide a method of manufacturing a tunable thin film capacitor comprising a dielectric layer with less lattice defects of oxygen, which need not be heat-treated for a long time, can be formed in a short time and with low costs.
Further another object of the invention is to provide a tunable thin film capacitor constituted such that, even in case that the upper electrode is made thick in order to reduce the loss of the electrode, the electrode is hard to peel off.
The present invention relates to a tunable thin capacitor characterized by comprising
a support substrate,
a lower electrode layer provided on the support substrate,
a thin film dielectric layer which is provided on the lower electrode layer and whose dielectric constant is changed by application of a control voltage, and
an upper electrode layer provided on the thin film dielectric layer,
wherein the thin film dielectric layer is held between the lower electrode layer and the upper electrode layer so as to form a plurality of capacitance-producing regions, and the respective capacitance-producing regions are electrically connected to each other.
The invention is further characterized in that the upper electrode layer on one of the mutually adjacent capacitance-producing regions is connected so as to be equipotential to the lower electrode layer of the other capacitance-producing region.
The invention is further characterized in that the plurality of capacitance-producing regions are an even number of capacitance-producing regions.
According to the invention, the tunable thin film capacitor is constituted such that the capacitance-producing region held between the upper electrode layer and the lower electrode layer is divided into a plurality of capacitance-producing regions, and the plurality of capacitance-producing regions thus divided are connected to each other.
More specifically, in case it is assumed that the thickness of the thin film dielectric layer is fixed, and the whole area of the capacitance-producing region necessary to produce a predetermined capacitance is fixed, this capacitance-producing region is divided into a plurality of capacitance-producing regions, and yet, they are joined with each other.
Accordingly, the area of each of the individual capacitance-producing regions is reduced, as a result of which the area of each of the individual mutually facing portions of the upper electrode layer and the lower electrode layer is also reduced. In other words, in case that a predetermined external control voltage is applied across the plurality of upper electrodes and lower electrodes, the voltage drop due to the conductor resistance can be suppressed in the plane portions of the electrodes effectively by an amount corresponding to the area reduction of the respective electrode layers.
The plurality of divided capacitance-producing regions can be electrically connected in parallel to each other, and thus, the lower electrodes of the mutually adjacent capacitance-producing regions can be made equipotential to each other, and the upper electrodes can be made equipotential to each other. Further, the plurality of divided capacitance-producing regions can be electrically connected in series to each other; and thus, the lower electrodes of the mutually adjacent capacitance-producing regions can be made different in potential from each other, and the upper electrodes can be made different in potential from each other.
Further, the planar shape of one of the upper and lower electrode layers, for example the planar shape of the lower electrode layer, is in a relationship of similitude to the planar shape of the capacitance-producing region. The area of the capacitance-producing region is determined depending on the mutually facing areas of the upper electrode layer and the lower electrode layer. This means that, for example, since the planar shape of the lower electrode layer is similar to the planar shape of the capacitance-producing region, there exists a portion, in the planar shape of the lower electrode layer, for which the upper electrode layer is not formed; and thus, the shape of the lower electrode layer is larger than that of the upper electrode layer. Accordingly, even in case that a positional offset is caused in the formation of both electrode layers, the positional divergence or offset is absorbed in case the positional offset is caused in the region in which the lower electrode layer and the thin film dielectric layer are formed; and thus, the capacitance characteristic is not varied. This structure is applied not only to the lower electrode layer side but also to the upper electrode side.
Further, in either case, the lower electrode layer is formed by deposition on the support substrate, the thin film dielectric layer is formed by deposition on the lower electrode layer and the support substrate, the upper electrode layer can be formed by deposition on the support substrate and the dielectric layer. Since the air bridge structure is not used unlike in the case of the known technique, the manufacturing steps can be performed by using substantially the thin film method. Further, the capacitor according to the invention structurally has no hollow structure portion in the structure thereof, so that the structural reliability can be enhanced, and the capacitor can be mounted on a mother board easily and infallibly by using the upper surface side of the support substrate as a mounting surface.
Further, the current due to the external control voltage dispersedly passes through the plurality of capacitance-producing regions, so that the parasitic inductance can be reduced. As a result, the self-resonant frequency defined as fo=xc2xd (LC)1/2 can be enhanced, and the frequency region in which the capacitor operates as such can be shifted to the high-frequency side.
In the tunable thin film capacitor according to the invention, the capacitance-producing region in which a predetermined capacitance is formed is divided into a plurality of capacitance-producing regions, and in addition, the thus divided capacitance-producing regions are electrically connected to each other. In other words, when a predetermined potential is applied across the upper electrode layer and the lower electrode layer holding the thin film dielectric layer therebetween to control the dielectric constant of the thin film dielectric layer, the potential distribution of the applied external voltage in the respective divided capacitance-producing regions can be decreased, so that the range of change by control is wide; and thus, a variable capacitor is realized in which a stable capacitance is obtained.
Further, current can be made to flow through each of the plurality of capacitance-producing regions, so that the parasitic inductance can be reduced.
Further, the directions in which the currents flow through in the mutually adjacent capacitance-producing regions are made to be the same direction, so that the magnetic fluxes produced by the currents can cancel each other, and thus, the change in the electrostatic capacitance can be further reduced.
Further, the shape of one electrode layer is made larger than the shape of the capacitance-producing region (the shape of the mutually facing areas of one electrode layer and the other electrode layer), so that, even in case that a positional of set is caused when one or the other electrode layer is formed, the variation in the area of the capacitance-producing region can be reduced, and thus, a stable characteristic can be obtained.
Further, no hollow structure exists between the respective electrode layers and the terminal electrode layers, so that the capacitor can be easily and infallibly mounted onto the mother board by using the upper surface side of the support substrate as a mounting surface.
The invention is characterized in that the lower electrode layer includes a first thin film substratum conductor layer formed by deposition on the support substrate, and a first thin film superstratum conductor layer formed by deposition on the first thin film substratum conductor layer, the upper electrode layer includes a second thin film substratum conductor layer formed by deposition on the support substrate in a state separated from the first thin film substratum conductor layer, and a second thin film superstratum conductor layer formed by deposition on the second thin film substratum conductor layer so as to face the first thin film substratum conductor layer,
the thin film dielectric layer is formed by deposition so as to extend over the first thin film substratum conductor layer and the second thin film substratum conductor layer,
the first thin film superstratum conductor layer is formed by deposition so as to cover a portion of the thin film dielectric layer, and,
by a portion of the first thin film substratum conductor layer and a portion of the second thin film superstratum conductor layer, the thin film dielectric layer is held therebetween to thereby constitute the capacitance-producing region.
In this way, one capacitance electrode, which is the lower electrode layer for example, of the capacitance electrodes that hold the thin film dielectric layer therebetween is comprised of the first thin film substratum conductor layer and the first thin film superstratum conductor layer. Further, the other capacitance electrode, which is the upper electrode layer for example, is comprised of the second thin film substratum conductor layer and the second thin film superstratum conductor layer. Further, the capacitance-producing region of the thin film dielectric layer is the facing region through which the first thin film substratum layer and the second thin film superstratum conductor layer face each other.
Further, in the invention, the first thin film substratum conductor layer and the second thin film substratum conductor layer should desirably be comprised of a material composed mainly of Au. Further, the first thin film superstratum conductor layer and the second thin film superstratum conductor layer should also desirably be comprised mainly of Au.
In the invention, the thickness t1 of the first thin film substratum conductor layer and the second thin film substratum conductor layer should desirably be larger than the thickness t2 of the first thin film superstratum conductor layer and the second thin film superstratum conductor layer.
According to the invention, the means for applying an external voltage to the thin film dielectric layer and, also, for deriving the capacitance component are a first capacitance electrode and a second capacitance electrode. Either one of these capacitance electrodes comprises a laminated structure of a thin film substratum conductor layer and a thin film superstratum conductor layer. Thus, since each capacitance electrode is of this laminated structure, it becomes easy to lower the resistance of the capacitance electrode itself. Further, since the thickness of the capacitance electrode is fixed and the thickness of the thin film substratum conductor layer can be made extremely small, the adherence between the thin film substratum conductor layer and the support substrate can be enhanced. Further, the adherence between the thin film substratum conductor layer and the thin film dielectric layer can be maintained, and in addition, the occurrence of a step coverage defect of the thin film dielectric layer at the ridgeline of the thin film substratum conductor layer can be prevented.
No air bridge is formed unlike in the case of the known thin film capacitor, and the short-circuiting between the two capacitance electrodes can be prevented. Further, since the capacitance electrodes are structured minutely, bump terminals, etc. can be easily formed, and further, in the case of mounting this element on a mother board, the mounting thereof can be easily performed.
Further, Au is used as the material of the thin film substratum conductor layer and the thin film superstratum conductor layer, so that the formation of an oxidation-preventing layer becomes unnecessary since Au is a non-oxidizing metal; and thus, the conductor loss of the capacitance electrodes can be further reduced. Moreover, since the thin film substratum conductor layer and the thin film superstratum conductor layer are made of the same metal material, the two conductor layers well adhere to each other when they are laminated.
According to the invention, the tunable thin film capacitor is constituted such that thin film dielectric layers are formed separated from each other so as to extend over the first thin film substratum conductor layer and the second thin film substratum conductor layer, and further, the first thin film superstratum conductor layer is disposed on the first thin film substratum conductor layer, while the second thin film superstratum conductor layer is disposed on the second thin film substratum conductor layer. In other words, the first capacitance electrode is formed of the first thin film substratum conductor layer and the first thin film superstratum conductor layer, and the second capacitance electrode is formed of the second thin film substratum conductor layer and the second thin film superstratum conductor layer.
Due to this structure, the adherence between the first thin film substratum conductor layer and the first thin film superstratum conductor layer and the adherence between the second thin film substratum conductor layer and the second thin film superstratum conductor layer are enhanced, and, even in case that the thickness of the first thin film superstratum conductor layer and the second thin film superstratum conductor layer is increased, the separation by peeling of the various conductor layers and thin film dielectric layers can be prevented. Further, by using Au as the respective materials of the first thin film substratum conductor layer, the second thin film substratum conductor layer, the first thin film super stratum conductor layer, and the second thin film superstratum conductor layer, the conductor loss of the capacitance electrodes can be reduced.
Further, since the resistance of the capacitance electrodes is lowered, the distribution of potential of the capacitance electrodes in the capacitance-producing region can be suppressed, and thus, a variable capacitance element that can withstand the application of an external voltage and is thus very high in practical utility can be realized.
The present invention is characterized in that a thickness of the thin film dielectric layer is 0.5 to 3.0 times as large as the thickness of the first and second thin film substratum conductor layers.
The invention is further characterized in that thicknesses of the first and second thin film superstratum conductor layers are 0.5 to 10 times as large as the thickness of the first and the second thin film substratum conductor layers.
According to the invention, one of the capacitance electrodes that hold the thin film dielectric film therebetween is comprised of a first thin film substratum conductor layer and a first thin film superstratum conductor layer, while the other capacitance electrode is comprised of a second thin film substratum conductor layer and a second thin film superstratum conductor layer.
According to the invention, one capacitance electrode can be comprised of a thin film substratum conductor layer and a thin film superstratum conductor layer, so that, even in case that the thickness of the thin film substratum conductor layer is made to be the same as the thickness of the known lower electrode layer, and the thickness of the thin film superstratum conductor layer is made to be the same as the thickness of the known upper electrode layer, the effective thickness of the capacitance electrode is increased, so that the loss of the capacitance electrode portion is reduced. Further, even in case that only the thickness of the thin film superstratum conductor layer is increased, the peeling of the layer is hard to take place since it closely adheres to the thin film substratum conductor layer.
On the other hand, the thinner the thin film substratum conductor layer is, the better the coverage of the thin film dielectric layer with respect to the thin film substratum conductor layer is; and, in the structure according to the invention, the thickness of the thin film substratum conductor layer can be positively reduced. Further, by setting the thickness of the thin film dielectric layer to a value that is 0.5 to 3.0 times as large as the thickness of the thin film substratum conductor layer, it is possible to obtain such a coverage that no step coverage defect exists in the stepped portion of the thin film dielectric layer disposed between the pair of thin film substratum conductor layers. In case that the thickness of the thin film dielectric layer is made less than 0.5 times as large as the thickness of the thin film substratum conductor layers, the thickness of the thin film dielectric layer becomes relatively small as compared with the thickness of the thin film substratum conductor layers, and thus, the coverage of the thin film dielectric layer lowers. Conversely, in case that the thickness of the thin film dielectric layer is made larger than 3.0 times as large as the thickness of the thin film substratum conductor layers, the problem pertaining to the coverage of the thin film dielectric layer is solved, but it takes a great deal of time to form the thin film dielectric layer, and thus, the productivity deteriorates.
Further, by setting the thickness of the thin film superstratum conductor layers to 0.5 to 10 times as large as the thickness of the thin film substratum conductor layers, the loss in the capacitance electrode portions can be reduced.
In case that the thickness of the thin film superstratum conductor layers is less than 0.5 times as large as the thickness of the thin film substratum conductor layers, the effect of reducing the loss of the capacitance electrodes is decreased, and at the same time, the thickness of the thin film superstratum conductor layers becomes relatively small, so that a step coverage defect takes place at the end of the portion covered by the thin film superstratum conductor layers and the thin film dielectric layer. In other words, the coverage of the thin film superstratum conductor layers lowers. Conversely, in case that the thickness of the thin film superstratum conductor layers exceeds a value that is ten times as large as the thickness of the thin film substratum conductor layers, the thickness of the thin film substratum conductor layers and the thin film superstratum conductor layers put together becomes too large, peeling becomes apt to take place in the interface therebetween. In connection with this, it is considered that, since a long deposition time is required for the formation of the thin film superstratum conductor layers, internal stresses such as the heat, etc. at the time of deposition are accumulated in the thin film superstratum conductor layers, and, due to these internal stresses, peeling is caused.
According to the invention, by setting the thickness of the thin film dielectric layer to a value that is 0.5 to 3.0 times as large as the thickness of the thin film substratum conductor layers, the coverage of the thin film dielectric layer is enhanced, the occurrence of a short-circuiting phenomenon between the first capacitance electrode and the second capacitance electrode can be prevented, and in addition, the productivity of the thin film dielectric layer can be maintained.
Further, by setting the thickness of the thin film superstratum conductor layers to a value that is 0.5 to 10 times as large as the thickness of the thin film substratum conductor layers, the effective thickness of the electrodes can be increased to reduce the loss in the capacitance electrode portion, and in addition, the occurrence of circuit-opening in the upper electrode layer can be prevented; and thus, the adherence of both electrodes can be maintained.
The present invention is characterized in that the lower electrode layer comprises a thin film substratum conductor layer formed by deposition on a support substrate and an upper conductor layer formed so as to project, in correspondence to the capacitance-producing region, from the thin film substratum conductor layer, and,
around the upper conductor layer, an insulator layer is disposed so as to be substantially flush with at least the surface of the upper conductor layer.
According to the invention, the lower electrode layer comprises a thin film substratum conductor layer formed in a state closely adhering to the support substrate and an upper conductor layer which is formed in a state projecting, in correspondence to the capacitance-producing region, from the thin film substratum conductor layer and which is smaller in area than the thin film substratum conductor layer for example and closely adheres to a thin film dielectric layer to define the capacitance-producing region.
As a result, the thickness of the capacitance-producing region of the lower electrode layer can be increased, and the electrode loss can be reduced. Further, by the region of the upper conductor layer, the capacitance-producing region can be defined, so that, by the formation of the upper conductor layer, the control of the capacitance component becomes possible, and thus, the area of the capacitance-producing region can be accurately formed.
Further, by forming, around the upper conductor layer, an insulator layer that is substantially flush with the surface of the upper conductor layer, the occurrence of step coverage defects can be prevented in the case of forming the thin film dielectric layer on the upper conductor layer and in the case of forming the upper electrode layer on the thin film dielectric layer. In addition, the thickness of the upper electrode layer can be easily increased, and therefore, the electrode loss of the upper electrode layer can be effectively suppressed. Further, the thin film substratum conductor layer and the upper conductor layer of the lower electrode layer can also be formed integrally.
By composing the thin film dielectric layer by the use of (BaxSr1-x) TiO3, it is also possible to realize a tunable thin film capacitor whose dielectric constant is changed by the application of an external voltage. In particular, a DC bias is applied across the lower electrode layer and the upper electrode layer, but, by increasing the electrode thickness in the portion of the capacitance-producing region, the non-uniformity of the bias is not caused, and thus, a stable control for the dielectric constant becomes possible.
Thus, according to the invention, as a thin film capacitor, a capacitor with a small capacitance for example can be easily fabricated, and, even in case that thick electrodes are used, the occurrence of steps which deteriorates the leakage characteristic of the dielectric portion and is caused during the manufacturing steps can be prevented; and further, depending on the area of the lower electrode that is contacted with the dielectric, the capacitance of the capacitor is substantially determined, so that the accuracy in positioning matching at the photolithography step is enhanced.
According to the invention, the tunable thin film capacitor is a thin film capacitor constituted such that, a lower electrode layer, a thin film dielectric layer, and an upper electrode layer are successively deposited on a support substrate, wherein the lower electrode layer is comprised of two layers, i.e., a thin film substratum conductor layer and an upper conductor layer, and further, by the upper conductor layer, the capacitance-producing region is defined. In addition, around the upper conductor layer, an insulator layer is formed so as to become substantially flush with the surface of the upper conductor layer.
As a result, there is obtained a thin film capacitor in which no step coverage defect exists in the thin film dielectric layer and the upper electrode layer, and the thickness of the upper electrode layer and the lower electrode layer in the capacitance-producing region can be increased as much as possible, so that the electrode loss can be effectively suppressed.
In particular, since the capacitance of the capacitor is almost determined depending on the area of the substratum conductor layer, a very high accuracy is not required in the positional matching at the photolithography step; and thus, a stable capacitance characteristic can be obtained without fail.
In particular, in the tunable thin film capacitor in which the dielectric constant of the thin film dielectric layer is changed by applying a DC bias across the lower electrode layer and the upper electrode layer, the potential dispersion can be suppressed within the electrodes.
The present invention is characterized in that the lower electrode layer includes an adherent layer formed by deposition on the support substrate, a thin film conductor layer formed in a state closely adhering to the adherent layer, and a buffer layer formed by deposition on the thin film conductor layer, wherein
the thin film conductor layer is comprised substantially of gold.
The invention is further characterized in that the adherent layer and the buffer layer comprise at least one of platinum, palladium and rhodium.
Further, in the invention, the thickness of the buffer layer is desirably at least 30 nm but not more than 0.5 xcexcm and 6% or higher with reference to the thickness of the thin film conductor layer.
Further, in the invention, the thickness of the adherent layer is at least 5 nm but not more than 0.5 xcexcm and 12% or higher of the thickness of the buffer layer.
According to the invention, a tunable thin film capacitor is constituted such that a lower electrode layer, a thin film dielectric layer and an upper electrode layer are successively deposited on a support substrate, and the lower electrode layer is constituted such that an adherent layer comprised of at least one of platinum, palladium and rhodium, a thin film conductor layer comprised of gold that is a metal with a high conductivity, and a buffer layer comprised of at least one of platinum, palladium and rhodium are successively laminated on the support substrate.
The buffer layer possesses the effect of suppressing the change in properties and oxidation of the thin film conductor layer due to the high temperature during the manufacturing steps and, in addition, preventing the reaction of the thin film conductor layer to the thin film dielectric layer.
The thin film conductor layer enhances the conductivity of the whole lower electrode layer and increases the Q-value of the capacitor.
Further, the adherent layer enhances the adherence between the support substrate and the thin film conductor layer, cancels out the thermal stress produced by the buffer layer and prevents the occurrence of inconveniences such as the peeling of the substratum conductor layer and the thin film dielectric layer as well as the peeling of the lower electrode layer from the support substrate during the manufacturing steps.
As the material of the thin film conductor layer, gold is used from the viewpoint that gold well adheres to the materials constituting the buffer layer and adherent layer and, further, low in reactivity to the dielectric material.
Further, the thickness of the buffer layer is at least 30 nm but not more than 0.5 xcexcm, and the buffer layer has a thickness that is 6% or more of the thickness of the thin film conductor layer. In case that the thickness of the buffer layer is less than 30 nm and, in addition, less than 6% with reference to the thickness of the thin film conductor layer, the change in properties and the oxidation of the thin film conductor layer and the reaction of the thin film conductor layer to the thin film dielectric layer can not be effectively prevented. Further, in case that the thickness of the buffer layer is more than 0.5 xcexcm, the thickness of the buffer layer becomes relatively large, so that the internal stress at the time of forming the buffer layer by deposition tends to become large. As a result, the peeling of the electrode becomes noticeable. Further, since the buffer layer is made of a material having a conductivity lower than that of gold which is the material of the thin film conductor layer, the electrode loss of the lower electrode layer as a whole tends to increase.
In particular, in case that the thickness of the buffer layer is not 6% or higher of the thickness of the thin film conductor layer, the electrode surface is roughened due to the change in properties of the thin film conductor layer due to the high temperature during the manufacturing steps, as a result of which leakage current, short-circuit and the reaction of the buffer layer to the thin film dielectric layer are caused more.
Further, the thickness of the adherent layer is at least 5 nm but not more than 0.5 xcexcm, and the adherent layer has a thickness that is 12% or more with reference to the thickness of the buffer layer. In case that the thickness of the adherent layer is less than 5 nm, the adherence of the adherent layer to the support substrate is deteriorated, and thus, inconveniences such as the peeling of the lower electrode layer, etc. are caused. Further, in case that the thickness of the adherent layer is more than 0.5 xcexcm, the thickness of the adherent layer becomes relatively large, and thus, the internal stress at the time of forming the adherent layer by deposition tends to increase, as a result of which the peeling of the electrode becomes noticeable. Further, since the material of the adherent layer is low in conductivity as compared with that of gold that is the material of the thin film conductor layer, the electrode loss of the whole lower electrode layer tends to increase. In case that the thickness of the adherent layer is lower than 12% of the thickness of the buffer layer, the stress caused by the buffer layer cannot be cancelled out, and thus, inconveniences such as the electrode peeling of the lower electrode layer, etc. are caused.
According to the present invention, a tunable thin film capacitor is constituted such that a lower electrode layer, a thin film dielectric layer, and an upper electrode layer are successively deposited on a support substrate, and the lower electrode layer is constituted such that an adherent layer comprised of at least one of platinum, palladium and rhodium, a thin film conductor layer comprised of gold that is a metal with a high conductivity, and a buffer layer comprised of at least one of platinum, palladium and rhodium are successively deposited on the support substrate. Thus, there can be provided a thin film capacitor in which the loss in the lower electrode layer is small, particularly, even at a high frequency, and therefore, the Q-value is high, and the lower electrode layer is hard to peel off during the manufacturing steps; and thus, the reliability is high.
The present invention is characterized in that there is included a protective film covering approximately the whole surface of the support substrate on which the lower electrode layer, the thin film dielectric layer and the upper electrode layer are formed, and
a region on the lower electrode layer and a region on the upper electrode layer are exposed from the protective film.
The invention is further characterized in that, on the lower electrode layer and on the upper electrode layer, an adherent layer is formed.
The invention is further characterized in that the adherent layer comprises platinum and/or palladium, and a thickness thereof is 0.1 to 1 xcexcm.
The invention is further characterized in that the adherent layer is exposed from the protective layer, and, on the thus exposed adherent layer portions, external terminal electrodes are formed.
The invention is further characterized in that the outer peripheral portion of the protective layer adheres to the peripheral portion of the surface of the support substrate.
According to the invention, a tunable thin film capacitor comprises a lower electrode layer formed on a support substrate, a thin film dielectric layer formed on the lower electrode layer, an upper electrode layer comprised of a low-resistance conductor and formed on the thin film dielectric layer, an adherent layer comprised of Pt and/or Pd and formed on the upper electrode layer, solder bumps formed as external terminal electrodes on the adherent layer and a protective film.
The tunable thin film capacitor can be mounted on a mother board easily and infallibly through solder bumps by using the upper surface of the support substrate as a mounting surface, and the loss due to the unnecessary electrode length and the self-inductance as in the case of the air bridge structure can be reduced. Due to this, the self-resonant frequency is high, and the capacitor is operable even in a high-frequency region.
Further, as the material of the upper electrode layer, a low-resistance conductor such as, e.g., Au is used, so that the electrode loss can be further reduced.
Further, on this upper electrode layer, the adherent layer comprised of Pt and/or Pd is formed, so that the adherence of the protective film formed on the adherent layer is good, and, by the protective film, the occurrence of cracks and peeling can be prevented. Due to this, a tunable thin film capacitor with a high reliable can be realized. Accordingly, by the provision of this adherent layer, the reliability in moisture resistance of the protective film is enhanced, whereby the migration of a low-resistance conductor such as, e.g., Au or Ag can be suppressed. In connection with this, it is pointed out that, in the case of a tunable thin film capacitor, a predetermined DC voltage is applied across the upper electrode layer and the lower electrode layer to control the dielectric constant of the dielectric layer, and thus, migration is more ready to be caused as compared with an ordinary thin film capacitor. However, due to the provision of the adherent layer, particularly the occurrence of migration can be suppressed, and the insulation resistance can be well maintained.
The present invention is characterized in that the thin film dielectric layer comprises perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, and the crystal grains are orientated in a plane (111).
The invention is further characterized in that, in the perovskite-type oxide constituting the thin film dielectric layer, x in (BaxSr1-x) TiO3 is in a range of 0.4 to 0.6.
The invention is further characterized in that the thickness of the thin film dielectric layer is 1 xcexcm or less, the dielectric crystal grains constituting the thin film dielectric layer are columnar grains that are long in the film thickness direction, and an average grain diameter in the film surface direction of the crystal grains is smaller than 0.5 xcexcm.
The invention is further characterized in that the lower electrode layer contacted with the thin film dielectric layer comprises Pt, Au or a solid solution thereof that is orientated in the plane (111).
The invention is further characterized in that the thin film dielectric layer is composed of a lower thin film dielectric layer having a thickness that is xc2xd or less of the thickness of the thin film dielectric layer and an upper thin film dielectric layer being the remainder, wherein, after the lower thin film dielectric layer is formed by deposition, the lower thin film dielectric layer is orientated in the plane (111) by heat-treating.
The invention is based on the discovery of the fact that the lattice defects of oxygen resulting from the heat treatment performed after the sputtering of a BST thin film being a thin film dielectric layer can be suppressed in a short time in case that the thickness of the BST thin film is sufficiently small. Further, this thin layer in which the lattice defects of oxygen have been suppressed is used as a lower dielectric layer, and the crystal structure grows columnar in the film thickness direction. By so doing, a thin film dielectric layer can be obtained in which the lattice defects of oxygen are less and the dielectric loss has been suppressed as compared with the case that the thin film dielectric layer is deposited directly on the lower electrode layer. More specifically, on the support substrate on which the thin film dielectric layer is to be formed, the lower electrode layer comprised of Pt, Au or a solid solution thereof is formed by sputtering, whereby these metals that are of the face centered cubic lattice structure are orientated in the (111) plane. In this state, the lower dielectric layer of BST formed on this layer can also be orientated in the (111) plane.
According to the invention, the tunable thin film capacitor is constituted such that the thickness of the thin film dielectric layer is 1 xcexcm or less, the thin film dielectric layer is comprised of perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, and the crystal grains are orientated in the (111) plane. Further, the dielectric crystal grains constituting the thin film dielectric layer are columnar grains that are long in the film thickness direction, and the average grain diameter in the film surface direction of the crystal grains is smaller than 0.5 xcexcm. Such a thin film dielectric layer is contacted with the lower electrode layer on the support substrate and is comprised of Pt, Au or a solid solution thereof orientated in the (111) plane.
Thus, a tunable thin film capacitor that comprises a thin film dielectric layer having a small number of lattice defects of oxygen can be provided, and in addition, the tunable thin film capacitor having a thin film dielectric layer having a small number of lattice defects of oxygen can be manufactured without the necessity of performing heat treatment extending over a long time and thus in a short time and at a low cost.
The capacitor according to the invention is a tunable thin film capacitor comprising a thin film dielectric layer with a very little loss due to very small number of lattice defects of oxygen provided to the whole of the thin film dielectric layer.
Further, a thin film dielectric layer with a small number of lattice defects of oxygen can be formed without the necessity of a heat treatment extending over a long time, that is, in a short time, at a low cost and easily.
The present invention is characterized in that the thin film dielectric layer comprises perovskite-type crystal grains containing at least Ba, Sr and Ti, and a composition thereof is represented as (BaSr) TixO3, wherein x in the composition is in a range of 0.7 to 0.9.
The invention is further characterized in that the perovskite-type oxide crystal grains constituting the thin film dielectric layer are orientated in a plane (110).
The invention is further characterized in that the thin film dielectric layer is formed on a lower electrode layer comprising Pt, Au or a solid solution thereof orientated in the plane (111) on the support substrate, by the sputtering process at a substrate temperature of 400xc2x0 C. to 600xc2x0 C.
According to the invention, in the thin film dielectric layer of the tunable thin film capacitor, the range of x in the perovskite-type oxide, (BaSr) TixO3 is 0.7 to 0.9. By setting the range of x to 0.7 to 0.9 and shifting to the side at which Ti is less, the composition of the thin film of BST is also shifted from the stoichiometric composition to the side at which Ti is less, whereby the lattice defects of oxygen in the thin film dielectric layer can be suppressed, and the increase in the dielectric loss due to the these lattice defects of oxygen can be suppressed. In case that the range of x is less than 0.7, the deviation from the specified composition range is too large to maintain the perovskite-type structure. Further, in case that the value of x exceeds 0.9, the effect due to the decrease of Ti is small, and the loss increases.
Further, the substratum electrode layer is formed by sputtering Pt, Au or a solid solution thereof, and these metals that are of the face centered cubic lattice structure are orientated in the (111) plane. Further, on the substratum electrode layer that is orientated in the (111) plane, a BST layer that is the thin film dielectric layer is formed by sputtering at a low temperature of 400xc2x0 C. to 600xc2x0 C., whereby the BST layer is orientated in the (110) plane.
In general, a thin film of BST that is formed at a low temperature contains a large amount of lattice defects of oxygen, so that it is a common practice to heat-treat the BST thin film in an oxygen atmosphere at a high temperature for a long time, but, according to the invention, the range of x in the composition of the target used for the sputtering is set to 0.7 to 0.9 in the stoichiometric composition, and the composition is deviated toward the side at which Ti is less, and the composition of the BST thin film is also deviated from the stoichiometric composition toward the side at which Ti is less, whereby the increase in the dielectric loss due to the lattice defects of oxygen is suppressed. Thus, a tunable thin film capacitor that comprises a thin film dielectric layer that has a small number of lattice defects of oxygen is realized; that is, a tunable thin film capacitor that comprises a thin film dielectric layer that exhibits a stable dielectric constant and a large rate of change of the dielectric constant with respect to the application of an external control voltage is realized.
Further, there can be realized a method of manufacturing a tunable thin film capacitor comprising a thin film dielectric layer that need not be heat-treated extending over a long time, that is, can be formed in a short time and at a low cost and yet has a small number of lattice defects of oxygen.
In the invention, the thickness of the thin film dielectric layer is 1 xcexcm or less, the thin film dielectric layer is comprised of perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, the composition of the crystal grains is represented as (BaSr) TixO3, the range of x in the composition is 0.7 to 0.9, the crystal grains are orientated in the (110) plane, the dielectric crystal grains constituting the dielectric layer are columnar grains that are long in the film thickness direction, and the average grain diameter in the film surface direction of the crystal grains is smaller than 0.5 xcexcm.
According to the invention, the thin film dielectric layer is disposed on a lower electrode comprised of Pt, Au or a solid solution thereof orientated in the (111) plane and is comprised of perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, wherein the range of x in the (BaSr) TixO3 is 0.7 to 0.9, and the crystal grains are orientated in the (110) plane. Accordingly, the deterioration in the dielectric loss due to the lattice defects of oxygen or the generation itself of the lattice defects of oxygen can be suppressed. Further, a heat treatment extending over a long time which was needed in the case of the known technique is not needed any more, and thus, a large quantity of tunable thin film capacitors can be fabricated with high efficiency and yet at low costs.
The invention is characterized in that the thin film dielectric layer comprises perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, and the crystal grains are orientated at random.
The invention is further characterized in that x in the perovskite-type oxide, (BaxSr1-x) TiO3, constituting the thin film dielectric layer is in a range of 0.4 to 0.6.
The invention is further characterized in that a thickness of the thin film dielectric layer is 1 xcexcm or less, and the dielectric crystal grains constituting the thin film dielectric layer are equiaxial grains, and an average grain diameter of the crystal grains is 0.5 xcexcm or less.
The invention is further characterized in that the thin film dielectric layer is constituted such that, on the lower electrode layer, an amorphous dielectric layer is deposited, and then the amorphous dielectric layer is crystallized by heat treatment to orientate the crystal grains at random.
The random orientation means a state in which, though the crystal grains exhibit some orientation in the crystal grain level, the crystal grains are not orientated in any specific plane as viewed from the whole thin film dielectric layer.
As a result of the examination ardently made by the present inventor, it was found that, by sputtering the dielectric layer at a low temperature of 200xc2x0 C. or below, an amorphous dielectric layer can be formed, and, thereafter, by crystallizing the amorphous dielectric layer by heat treatment, oxygen can be supplemented in the amorphous state in which the oxygen is more apt to diffuse than in the crystal, so that the suppression of the lattice defects of oxygen is made possible by heat treatment for a short time, and thus, the loss is decreased.
According to the invention, the thickness of the thin film dielectric layer is 1 xcexcm or less, the thin film dielectric layer is comprised of a perovskite-type oxide crystal particles containing at least Ba, Sr and Ti, and the crystal grains are orientated at random.
Further, according to the invention, the dielectric crystal grains constituting the thin film dielectric layer are equiaxial, and the average grain diameter in the film surface direction of the crystal grains is smaller than 0.5 xcexcm. Further, according to the invention, on the support substrate on which the thin film dielectric layer is to be formed, a lower electrode layer is formed, and the lower electrode layer is comprised of Pt, Au or a solid solution thereof orientated in the (111) plane.
As a result, a tunable thin film capacitor comprising a thin film dielectric layer in which the lattice defects of oxygen are small in number and the dielectric loss is small can be realized; a tunable thin film capacitor constituted such that, by application of a control voltage from outside, a predetermined electrostatic capacitance can be obtained can be realized.
Further, a method of manufacturing a variable capacitor comprising the dielectric layer that does not require a heat treatment extending over a long time and has a small number of lattice defects of oxygen, can be realized in a short time and at a low cost.
According to the invention, the tunable thin film capacitor is constituted such that the dielectric layer is formed in such a manner that, on a lower electrode layer comprised of Pt, Au or a solid solution thereof and orientated in the (111) plane, an amorphous dielectric layer is formed by sputtering at a low temperature and then crystallized by a heat treatment for a short time, where by a dielectric layer that is not orientated in any specific plane orientation is formed. Accordingly, the deterioration of the dielectric loss due to the lattice defects of oxygen that has so far caused or the generation itself of the lattice defects of oxygen can be suppressed. In addition, a large quantity of variable capacitance capacitors can be fabricated efficiently and at low costs without the necessity of performing heat treatment extending over a long time.
The present invention is characterized in that the thin film dielectric layer comprises a dielectric layer with a high dielectric constant that is positioned at the lower electrode layer side and comprises perovskite-type oxide crystal grains containing at least Ba, Sr and Ti and a dielectric layer with a low dielectric constant that is positioned at the upper electrode layer side and comprises oxide grains containing Ba and/or Sr.
The invention is further characterized in that an interface of the thin film dielectric layer with a low dielectric constant with the upper electrode layer assumes a projected/depressed shape due to the crystal grain diameters of the oxide grains.
The invention is further characterized in that the thin film dielectric layer with a low dielectric constant is distributed in an island-like state or in a reticulate state on the thin film dielectric layer with a high dielectric constant.
According to the invention, the thin film dielectric layer is constituted such that a dielectric layer with a low dielectric constant comprised of oxide grains containing Ba and/or Sr is disposed at the side of the upper electrode layer. The surface of this dielectric layer with a low dielectric constant comprised of oxide grains containing Ba and/or Sr is made into an uneven or projected/depressed state due to the grain diameters of the oxide grains. Accordingly, the interface between the thin film dielectric layer and the upper electrode layer is made into an uneven or projected/depressed state, as a result of which the upper electrode layer adheres to the thin film dielectric layer so as to structurally bite wedge-wise in to the depressed portions of the uneven or projected/depressed surface of the thin film dielectric layer. Accordingly, the adherence between the thin film dielectric layer and the upper electrode layer is enhanced, as a result of which a thin film capacitor having a low loss in the upper electrode layer is realized. This structure is particularly suited for a tunable thin film capacitor constituted such that the thin film dielectric layer is comprised of perovskite-type oxide crystal grains containing Ba, Sr and Ti, and, by applying an external voltage across the lower electrode layer and the upper electrode layer, the dielectric constant of the thin film dielectric layer is changed, because the concentration of the voltage at the electrode portions can be alleviated.
According to the invention, the thin film dielectric layer is constituted such that the dielectric layer at the side of the lower electrode layer is comprised of a dielectric with a high dielectric constant, and the dielectric layer at the side of the upper electrode layer is comprised of a dielectric with a low dielectric constant. More specifically, the dielectric layer with a high dielectric constant is comprised of perovskite-type oxide crystal grains containing at least Ba, Sr and Ti, and the dielectric layer with a low dielectric constant is comprised of oxide grains containing at least Ba and Sr.
Particularly, the surface of the dielectric layer with a low dielectric constant is uneven or projected and depressed due to the crystal grain diameters of the oxide grains containing Ba and Sr. As a result, the upper electrode layer formed on the thin film dielectric layer closely adheres to the thin film dielectric layer so as to tightly engage with each other. Accordingly, the upper electrode layer is hard to peel off, and further, even in case that the thickness of the upper electrode layer is increased, the upper electrode layer is still hard to peel off, as a result of which the electrode loss can be reduced.
The invention is directed to a high frequency device which is characterized by having the above-described tunable thin film capacitor.